The use of portable electronic instruments is increasing as electronic equipment gets smaller and lighter due to developments in the high-tech electronics industries. Studies on lithium secondary batteries are actively being pursued in accordance with the increased need for a battery having high performance and large capacity for use as a power source in these portable electronic instruments. Such a lithium secondary battery, having an average discharge potential of 3.7 V (i.e., a battery having substantially a 4 V average discharge potential) is considered to be an essential element in the digital generation since it is an indispensable energy source for portable digital devices such as cellular telephones, notebook computers, camcorders, etc (i.e., the “3C” devices).
Also, there has been extensive research on batteries for effective safety characteristics such as preventing overcharge. When a battery is overcharged, an excess of lithium ions is deposited on a positive electrode, and an excess of lithium ions is also inserted into a negative electrode to make the positive and negative electrodes thermally unstable. An eruptive explosion may occur from a decomposition of the electrolytic organic solvent, resulting in a thermal runaway which can cause serious safety concerns.
To overcome the above problems, it has been suggested that an aromatic compound such as an oxidation-reduction additive agent (“redox shuttle”) may be added to the electrolyte. For example, U.S. Pat. No. 5,709,968 discloses a non-aqueous lithium ion secondary battery that prevents thermal runaway caused by an overcharge condition by using a benzene compound such as 2,4-difluoroanisole. U.S. Pat. No. 5,879,834 discloses a method for improving battery safety by using a small amount of an aromatic compound such as biphenyl, 3-chlorothiophene or furan, which is polymerized electrochemically to increase the internal resistance of a battery during unusual overvoltage conditions. Such redox shuttle additives increase the temperature inside the battery early due to the heat produced by the oxidation-reduction reaction, and close the pores of a separator through quick and uniform fusion of the separator to inhibit an overcharge reaction. The polymerization reaction of these redox shuttle additives consumes the overcharge current to improve battery safety.
However, since batteries with high capacity are required according to meet the needs of customers, the above additives for preventing overcharge are limited in their ability to satisfy the high level of safety requirements. Accordingly a novel additive and an electrolyte including the same to assure such safety requirement are desired.